how to paint on sculpts without retopo for idealization?
If you don't want to do any texturing, you can use vertex paint. It's like painting on the surface of your 3D model. It's not the most advisable method of coloring, texturing usually is, but if you don't want to have to deal with retopo and just want an idea of what your colors are going to be, vertex painting is an great tool. I'm not that great at teaching, but I do have a video that might help:

Vertex Paint Workflow in Blender 2.8

Hope I somewhat answered your question :)